The present invention relates to a packaging bag for powdery or granular materials or, more particularly, to a packaging bag which is safe from the danger of bursting by the internal air pressure caused when a great number of the bags filled with a powdery or granular material are stacked one on the other to a considerable height as in storehouses.
It may be too much to say that a variety of powdery or granular materials are transported, stored and sold as packaged in packaging bags of a suitable size with convenience in handling. Such powdery or granular materials include foodstuffs, e.g. cereal grans, flours, sugar and the like, fillers and other additives used in the processing of rubbers and plastics, e.g. carbon blacks, man-made fertilizers, Portland cement and others.
It is a growing recent trend in the materials for the bags that packaging bags made of a film or sheet of a synthetic plastic resin are used in place of traditional paper bags made, for example, of kraft paper.
Plastic bags for packaging have their own advantages in their beautiful appearance, high mechanical strengths, excellent moisture-proof property and other respects. Plastic bags are moisture-proof owing to the water resistance of the synthetic plastic resin per se and the low permeability of air and atmospheric moisture through the films or sheets of the plastic resins.
It is recently proposed and practiced that fillers and other additive materials used in the processing of rubbers and plastics are packaged in bags made of films of a rubber or plastic resin having compatibility with and not adversely influencing the properties of the rubber or plastic resin to which the additive material is blended so that the filler or additive contained in the bags can be introduced as such into the blending machine in which the rubber or plastic resin is under milling without opening the packaging bags. Such a way of handling is particularly advantageous when the powdery material in the bags has a noxious nature such as carbon black which may cause serious contamination of environment even with a very small amount scattered in handling because, although carbon blacks are usually formed in beads or granules, considerable amount of finely divided powdery carbon is always contained in the bag due to the pulverization of the granules or beads during transportaion and handling.
Contrary to the great advantages described above, packaging bags made of a plastic or a rubber film or sheet have a very serious defect not encountered in paper bags having good air permeability. This defect is especially apparent when a great number of the bags filled with, for example, carbon black, are stacked one on the other to a heap as is frequently seen in storehouses. In such a circumstance, the bags lying at or near the bottom of the heap receive a tremendous pressure by the weight of the bags lying upon them. Therefore, the air remaining in the bags is heavily compressed to a considerably high pressure so that the bags become very likely burst causing a heavy contamination of the environment by the powder, e.g. carbon black, ejected from the broken bags.
Furthermore, when a bag containing carbon black or other additives is charged without being opened into a Banbury mixer, mixing roller mill or other blending machine with a large volume of air remaining after the bag has been sealed by welding or other hermetical means, the bag becomes burst in the blending machine also to cause environmental contamination by the carbon black ejected from the broken bag.
To overcome the above described difficulties, an attempt is practiced in bags for man-made fertilizers in which the bags are provided with a number of tinly holes at a suitable portion thereof so that the air in the bag can escape through the holes when the bag receives an external compression. This method is, however, not applicable to the bags for carbon blacks or other very fine powdery materials since the particles of the powdery material may be ejected or escape through the holes to cause environmental contamination.